The lagging of magnetization behind the magnetizing force as the magnetic condition of a ferromagnetic material is changed, e.g. when applying an alternating external field, is called magnetic hysteresis. When a ferromagnetic sample that is initially demagnetized is subjected to an increasing external magnetic field H it reaches a particular flux density B.sub.sat at the maximum value of H. When the value of H is decreased again the decreasing flux density does not follow the path of increase but decreases at a rate less than that at which it rose. When H has reached zero again the value of B is not reduced to zero but to a value called the retentivity or remanence. The sample has retained a permanent magnetization. The value of B may be reduced to zero by reversing th magnetic field to negative and increasing its value to the so-called coercive force or coercivity. By further increasing H to negative values and then again reversing its direction a hysteresis loop as represented in FIG. 1 is completed.